1. Field of the Invention
The present invention generally relates to the manufacturing of integrated circuits. More specifically, the present invention relates to the implantation of aluminum in semiconductor substrates generally used in the microelectronics industry. Such an implantation is performed in a substrate typically made of silicon. Aluminum is then used, instead of boron, to achieve a P-type doping. An advantage of aluminum is that it diffuses approximately seven times faster than boron. The aluminum is extracted from a solid alumina element (Al203), attached in an arc plasma ionization chamber. The aluminum that is desired to be implanted is in ionic form, a precursor gas being injected in the ionization chamber to release ions from the alumina pellet.
2. Discussion of the Related Art
An example of an aluminum implantation device of the type to which the present invention relates is described in U.S. Pat. No. 5,497,005, which is incorporated herein by reference. The aluminum implantation may be performed without a post-acceleration, as is the case in the above-mentioned document, or with a post-acceleration of the generated ions. In one case as in the other, a same ionic plasma source may be used. For example, such a plasma generator is based on a ionization chamber in which a precursor gas is injected. A filament conducting a current (generally between 0 and 200 mA) and biased with a potential difference with respect to the chamber walls releases electrons, which ionizes the gas present in the chamber. This gas is then used to etch an alumina pellet housed in the chamber, to release aluminum ions into the plasma. The chamber is provided with an opening generally of small dimension so that particles can be extracted therefrom. In a post-acceleration system, these ionized particles are accelerated by an electric field to reach the substrate on which the aluminum is to be implanted. In fact, these particles are extracted from the ionization chamber by the extraction energy (generally between 0 and 80 kV), then accelerated by the post-acceleration energy (generally between 0 and 80 kV).
In an implantor, not only the energy level of the ionization chamber, that is, the current in the filament of this chamber and the arc current, is set, but also the magnetic fields enabling the deviations of some ions to select, essentially by their atomic mass, the ions that are desired to be present in the plasma used for the implantation. Accordingly, among the steps of setting an ionization source of an implantor, reference will be made to the intensity of a given ion in the plasma. For the same extraction energy, the respective concentrations of the different ions in the plasma are different from one another and, when the extraction energy varies, the respective concentrations of the different ions vary in the same proportions with respect to one another.
The selection of the implanted ion is essentially performed by magnetic means of deviation of the ion beam at the ionization chamber exit. The ion is thus selected by its atomic mass in a curved tunnel, the ions of different masses being more or less deviated than the selected ion, then trapped by cryogenic pumps or stopped by walls.
The structure and operation of implantation systems are well known and will not be detailed any further.
To implant aluminum, above-incorporated by reference U.S. Pat. No. 5,497,005 provides using, as a precursor ionizing gas, silicon tetrafluoride (SiF4). This fluorided gas can easily be ionized to generate ions Si+, Si++, SiF+, SiF2+, and SiF3+. The plasma obtained by ionization can then etch the aluminum oxide or alumina (Al2O3) to generate ions Al+, Al++, and Al+++, that join the plasma to be subsequently implanted in the semiconductor wafer. Among these ions, that which is selected by the setting of the implantor is aluminum (I) ion Al+, which is extracted most from the pellet.
A disadvantage of the use of silicon tetrafluoride as the ionization precursor gas is that, among the obtained silicon ions, silicon (I) ion Si+has an atomic mass of 28, which is close to that of aluminum (I) ion Al+, the atomic mass of which is 27.
When the atomic masses of the ions are close, the neighboring peaks of these two ions overlap in an implantation current/atomic mass characteristic. The higher the extraction energy, the wider the peaks at their base and the more they overlap. Due to the overlapping phenomenon of neighboring peaks, the two ions are often implanted instead of a single one. This is especially the case for silicon and aluminum by using silicon fluoride as a precursor gas. Further, in this example, the peak of the silicon (I) ion is much larger than that of the aluminum (I) ion so that, to avoid silicon ion currents that are too high, it is necessary to limit the current of aluminum ions, which adversely affects the implantation. This is particularly true for high-energy implantors, that is, implantors using a post-acceleration phenomenon.
Implantors can be divided into two large families according to whether the setting (adjusting) of the implantor on the compound that is desired to be implanted is performed manually or automatically. For manual machines, the setting may enable dissociating two relatively close peaks (for example, the peaks of ions Si+and of ion Al+) to implant a single compound. However, such a dissociation is only possible for currents of small magnitude. As a result, to obtain a relatively high given dose, typically 1.1016 atoms/cm2, it is necessary to submit the substrate to prolonged implantations. This is disadvantageous in terms of productivity. For automatic adjustment machines, the resolution, which is defined as the atomic mass at the top of the peak divided by the mass differential at mid-height, does not allow avoiding the implantation of the undesired compound. In particular, if an arc occurs in the ionization chamber, the automatic setting results in selecting the compound corresponding to the highest intensity peak. In the considered example, this leads to selecting the silicon (I) ion instead of aluminum.
It should further be noted that the implanted aluminum dose, which depends on the intensity of the beam as well as on the electric fields applied in extraction and post-acceleration is of particularly delicate setting when in presence of two ions of relatively close atomic mass.
The present invention aims at providing a novel method of aluminum ion generation, in particular for an implantation in a semiconductor wafer, which overcomes the disadvantages of conventional methods.
The present invention more specifically aims at providing a method that frees itself of the problems of atomic mass proximity between aluminum and silicon.
The present invention also aims at providing a solution that is valid whether the implantor is associated or not with a post-acceleration of the generated ions.
The present invention further aims at providing a solution that is valid for both implantors with a manual setting and an automatic setting of the magnetic fields and of the extraction energy.
To achieve these and other objects, the present invention provides a method of aluminum ion generation for an implantation in a semiconductor wafer, including of using nitrogen trifluoride (NF3) as a gas for ionizing a solid alumina element (Al2O3).
According to an embodiment of the present invention, the obtained aluminum plasma is submitted to a post-acceleration.
According to an embodiment of the present invention, the extraction energy of the aluminum ions depends on the intensity of nitrogen monofluoride ions (NF+) in the plasma.